The rotors and casings of flow engines (for instance, steam turbines, compressors, gas turbines) are generally designed on the basis of strength and/or weight, and housings of relatively small mass are opposite rotors of relatively high mass. The following characteristics result from this:
an optimal clearance of, for instance, a few hundredths of a millimeter can be provided for the radial, rotating and stationary guide blades only for a specific load condition. Other load conditions must be carried out with radial slot sizes which result from the above optimizing design. Namely, not only are there variations in size depending on load conditions, but also for cold slots and in the thermal transition behavior of the rotor and stator between two different loading conditions;
in the event of excessive differences in the thermal transition behavior it may be necessary to make the "optimum" radial slot larger in order to avoid, in certain transient conditions, a radial scraping of the rotating and guide blades with their respective opposed structure;
the above is also important in the construction and sizing of the slots of radial seals.
As a result of conventional construction, there are a number of disadvantages as noted hereafter:
loss of power and efficiency, or increased consumption of fuel for load conditions which differ from that for which the slot sizes were optionally designed;
loss of power and efficiency, or increased consumption of fuel during transition states or non-steady operating conditions;
suceptibility to compressor pumping, particularly upon acceleration.
In order to provide a margin of safety in light of the varying conditions, it is known from U.S. Pat. No. 4,329,114 to provide a radial slot control device, which is adjustable as a function of engine output parameters, for compressors of gas turbine engines. In the patent there is disclosed a construction in which air taken from a relatively "cold" region of the compressor is selectively diverted, by a flap control valve on the outer casing, to flow either in entirety or in part along the outside of the corresponding inner guide blade support structure, and therefore over an axial length from the region of air removal approximately to the last compressor stage.
In another system for controlling the clearance between blades and opposite structure for compressors of gas turbine engines, U.S. Pat. No. 4,338,061 shows a system operating predominantly electronically and including a mechanical control valve by which relatively cold air is bled for turbine cooling or control of internal leakage. The cold air is bled from a comparatively early compressor stage, for instance from the fifth compressor stage as a by pass flow mainly for control of size of the radial slot due to blade clearance. In this regard, the bleed point communicates both with an outer first flow path extending along the compressor housing up to the last compressor stage and with a second flow path extending parallel to the latter. The mechanical control valve controls a variable passage of air through both flow paths and thus provides cooling of variable intensity of the outer housing. The optimal position of the control valve for the specific operating condition is representative of the size of the radial slot actually required at the time, as calculated by a computer system, using pertinent engine parameters based on the difference between the actual temperature and the desired temperature of the housing as predetermined for the condition of particular operation.
The following disadvantages result from the systems of the two U.S. Patents described above:
an expensive electronic construction is required;
expensive air guides and control means are required;
a comparatively large increase in weight is obtained by the added structure;
a larger engine diameter is required, particularly in the region of the compressor;
it is necessary to tap off comparatively large amounts of compressed air which compromises the compression;
as a whole, considerable susceptibility to turbulence is produced.
From the journal "INTERAVIA," 2 (February), 1983, page 102, middle column, last paragraph, there is known, by itself, a so-called "active" slot control for the compressor of a gas-turbine engine by introduction of hot air into the corresponding compressor rotor.